Pressure vessel of metal and silicon monoxide layers



Spt. 9, 1969 J. H, VA GH ET AL 3,466,224

PRESSURE VESSEL OF METAL AND SILICON MONOXIDE LAYERS Filed March 2. 1966 2 Sheets-Sheet 1 Will/1111550111 zaz zeliqzz l aaia l )I/l/l/ll/l/l/l I NVENTDREI dDHN H. VAUEHN,deceased,

- 1 4 5w OEFIETTA r1..VAuanN,

I 5' Legal Representative HENRY HAHN av R P. m

AEENT Sept. 9, 1969 J- H. VAUGHN ET AL 3,466,224

PRESSURE VESSEL OF METAL AND SILICON MONOXIDE LAYERS Filed March 2, 1966- 2 Sheets-Sheet 2 INVENTURS JOHN H.VAUE|HN,deceased. BY: OERETTA H.VAuarm,

' Legal Representative HENRY HAHN av 3. m P Wee.

AEELNT United States Patent 3,466,224 PRESSURE VESSEL OF METAL AND SILICON MONOXIDE LAYERS John H. Vaughn, deceased, late of Bloomfield, N.J., by Ogretta H. Vaughn, legal representative, Worcester, Mass., and Henry Hahn, Fairfax, Va., assignors to 'Curtiss-Wright Corporation, a corporation of Delaware Filed Mar. 2, 1966, Ser. No. 534,283 Int. Cl. B32b 15/04; C23c 13/02; B44d 1/12 U.S. Cl. 161-165 2 Claims ABSTRACT OF THE DISCLOSURE A pressure vessel of high strength structural material approaching theoretical atomic strength, formed of a plurality of thin films of metal alternating with and metallurgical separated by thin films of silicon monoxide.

This invention relates to materials of higher ultimate tensile strength, yield strength, and Youngs modulus than conventional metals, and more particularly to composite laminar structures.

It has long been recognized that all metals in their common bulk state have a much lower strength than is theoretically possible, basing predictions on the interatomic bond strength of a perfect crystal of such metals. It is well established that the failure to realize the theoretical strength is due to the fact that metals produced in the mass do not have perfect crystal lattices throughout. Instead, imperfections such as point and line defects (dislocations) exist in common metal lattices, and such imperfections readily transfer to adjacent crystals, resulting in the formation of slip planes along which relative movement or ruptures may occur as a result of relatively low stresses. Such lattice imperfections appear to be inherent in the presently known methods of metal production, and multiply themselves automatically in metal masses.

It has recently become known that very thin films of metal produced by vapor deposition exhibit very high strengths, approaching the theoretically predictable values. Such thin films have previously usually been of the order of about 1000 to 5000 angstroms in thickness, that is, about 25 millionths of an inch to about 125 millionths. The precise reason for such increased strengths is not known. It has been theorized that there is a lower limiting grain size below which regeneration of dislocations cannot take place due to obstruction of dislocation loops by grain interfaces. It has also been postulated that the high ratio of surface to volume in thin films gives rise to a surface effect, possibly analogous to the surface tension of a liquid.

The only uses previously known for such thin films have been electrical, as in capacitors, or as current-carrying paths of the printed circuit type. Thin films have not been useful as structural materials, since it is not effective to increase the thickness of a film by continued deposition of metal, as it then takes on the microstructure of massive metals, subject to the same lattice imperfections and movement of crystalline dislocations as before. Neither has it been effective to use a stack of thin films as a laminar structure, since if the films are permitted to touch metallurgically, they become, because of interlaminar diffusion, in effect a solid structure with movement and multiplication of dislocations.

In the present invention it has been found that a laminar structure of thin films of metal may be built up with alternate layers of a separating material to prevent such dislocation movement and interlaminar diffusion. A composite structural material or objects of laminar structure may be built up directly from such alternating layers. Thus it becomes possible to provide a material with a higher ratio of strength to weight than heretofore known, or to provide objects such as pressure vessels and rocket casings having such a high ratio of strength to weight. The material and objects made therefrom have particular utility in devices for space exploration, where weight saving is a most important consideration.

It is necessary that the separating layers be formed of a material which may be vapor deposited, and having a predominantly amorphous structure, since materials with a crystalline structure would be subject to the same disadvantages as a metal. Such an amorphous material which is thermally stable at high tempertures is silicon monoxide. For composites intended to be used at lower temperatures, or capable of brief periods of exposure to temperatures of several hundred degrees Fahrenheit, the separating layers may be formed of various resin plastics, particularly silicone resins.

- It is therefore an object of this invention to provide a structural material of high strength.

It is another object to provide a composite high strength material having a laminar structure of thin films of metal alternating with layers of separating material.

A further object of the invention is the provision of objects formed of high strength materials of laminar structure of thin films of metal alternating with layers of separating material.

The foregoing objects and others will be readily understood on reading the following specification in connection with the drawings, in which FIG. 1 is a much enlarged semischematic cross-sectional view of the laminar structural material;

FIG. 2 is a schematic view of a method of forming a cylinder or sheet of the laminar structural material;

FIG. 3 is a view partially in cross-section of another method of forming a sheet of the composite laminar structural material; and

FIG. 4 is a perspective view of a vessel or rocket case made according to the invention, partially cut away to show the laminar structure.

In FIG. 1 there is shown generally a composite struc tural material 11, made up of alternate thin films 12 of vapor deposited metal separated by layers 13 of vapor deposited separating material. Very nearly any metal may be vapor deposited, and its choice will be governed by design considerations and the use intended. Iron, aluminum, chromium, nickel, and titanium all have high strength potentials and are particularly suitable owing to their relative ease of handling. Because of its relatively low vaporization temperature and high vapor pressure, aluminum is especially suitable for this use, having high strength in the thin film form.

The number of layers in the composite structure 11, and the thickness of individual layers, are shown only schematically in FIG. 1. The actual thickness of each thin film of metal is from about 400 to 1000 angstroms, or four millionths to one hundred-thousandths of a centimeter. The layers of separating material, whether resin plastic or silicon monoxide, are from about 50 to 400 angstroms in thickness. Therefore, with :a metal film thickness of about 600 angstroms and a separator thickness of about 200 angstroms, each composite pair of layers 12 and 13 will be about 800 angstroms thick, and about 125,000 such pairs are required to make a composite structural sheet one centimeter thick. For a composite sheet about .020 inch thick approximately 6300 such pairs of alternating metal film and separating film are required. Such a structural sheet material 11 has a tensile strength approximately 15 to times that of steel.

FIG. 2 shows a cylinder of composite structural material and a method of forming it, or of forming a sheet. Vaporization and deposition of the materials is performed at low pressure, generally about 10- mm. of mercury or lower. Accordingly, there is shown a vacuum tank or vessel 14 in which the process is carried out. Positioned within the vacuum tank are two elongated crucibles or boats 16 and 17 formed of a suitable high temperature material, such as tungsten, ceramic, or other refractory. The crucibles are arranged for heating by any suitable means, as the electrical coils 18, for example. If the bottoms of the crucibles are sealed into the wall of the vacuum tank, they may be heated by an external heat source.

A cylindrical mandrel 19 is positioned horizontally above the crucibles and rotatable on its longitudinal axis by any convenient means (not shown). The mandrel, having a smooth surface, is coated with a suitable release agent 21, such as collodion or a plastic which is readily soluble. Alternatively, the mandrel itself may be made of a soluble material, such as rock salt, in which case it is used without a release agent.

Each of the crucibles 16 and 17 has a slit-like orifice 22 and 23, respectively, generally parallel with the axis of the mandrel and positioned close to its surface. As shown in FIG. 2 the mandrel is arranged to rotate in a clockwise direction. On heating the crucibles to proper temperature, metal vapor generated in crucible 16 passes through orifice 2.2 and condenses in a thin film on the surface of the rotating mandrel. The portion of the mandrel bearing the thin metal film then passes over orifice 23 from which arises the vaporized material of the separating layer, silicon monoxide or a silicone resin. The temperatures of the two crucibles may be separately and variably controlled in order to provide the proper feed of vapor from each. It is unnecessary to cool the mandrel, since the vapor will condense and deposit instantly on a surface only slightly cooler than the vaporization temperature.

As the process continues there is deposited at each revolution of the mandrel a thin film of metal separated from the preceding by a layer of the selected separating material to form the cylinder 24. Cylinder 24 is in fact spirally wound with alternate layers, a matter of no significance, since there is no measurable deviation from concentricity.

If it is desired to retain the product 24 in cylindrical form, the mandrel carrying the deposit may on completion be soaked in a solvent to loosen the release agent, or to dissolve the mandrel if a soluble mandrel has been used. If a sheet of composite material is desired, it may be longitudinally slit through to the surface of the mandrel before removing it, which will materially speed up the process of loosening the release agent.

A sheet of composite material 11 may also be formed on a mandrel which is a flat plate rather than cylindrical, by arranging suitable automatic traversing means which will alternately traverse the plate across the orifices of two crucibles, one containing the selected metal and the other the selected separating material. In such a case there is arranged an automatic feed for each crucible, whereby finely divided material is fed into the respective crucibles at the proper rate and vaporized only when the plate is in a position to receive the condensate.

Further, it is sometimes desirable that a composite structural sheet be made of a plurality of thin films of different metals alternating with layers of separating material. To provide such a sheet the mandrel or substrate can be traversed over a series of crucibles providing vapors of different metals, positioned alternately with crucibles which provide the vapor of the separating layers. Such an arrangement is shown in FIG. 3.

A series of crucibles 16 for evaporating the various selected metals alternates with crucibles 17 for evaporating separating material, all positioned within a vacuum chamber 14a. A flat mandrel or substrate 19a coated with a release agent 21 is arranged to traverse reciprocally over the crucibles, presenting a flat side coated with the release agent to the orifices 22 from which metal vapor emanates and the orifices 23 from which the separating material is evaporated. The substrate 19a may travel along rails 28 by dolly means 29, as shown, or by any other convenient means, impelled by any conventional motive power (not shown). A feed means 31 is provided for each crucible, whereby selected metals and separating material are fed to each crucible in turn in synchronization with the passage of the substrate thereover. By this means a plurality of different metals can be evaporated in individual layers onto the substrate alternating with separating material, to provide a composite structural sheet 11 comprising many layers of difierent selected metals, metallurgically separated from each other. Although in FIG. 3 crucibles are shown for only three different metals, it will be understood that any desired number may be used.

In FIG. 4 there is shown a pressure vessel 26, which may be a rocket casing having an exhaust nozzle portion 27 as shown, or a pressure vessel for some other purpose. There is shown schematically an aperture cut through the wall of the vessel, merely to demonstrate the alternate layers of thin films 12 of metal and separating layers 13. Such a vessel, not being a right circular cylinder throughout, could not conveniently have a permanent mandrel removed from the interior. Therefore, it can be made on a soluble mandrel. Further, if the vessel is reasonably large, the vapors can be deposited on the inside of a rotatable partible mold, by arranging the crucibles on supporting members extending therein.

Although for convenience the crucibles have been referred to as being formed of tungsten, there are other re fractory materials which will withstand the necessary temperatures, such as graphite; hafnium and zirconium b0 rides; hafnium, columbium, and titanium carbides; and hafnium nitride. Following are the approximate vaporization temperatures of some of the metals to which the in vention is applicable. These are the approximate temperatures of vaporization at one atmosphere of pressure; when the pressure is reduced to 10 or lower vaporization proceeds rapidly at markedly lower temperatures.

Degrees F. Aluminum 3700 Chromium 4500 Iron 5430 Nickel 5250 Titanium 5900 Silicon monoxide vaporizes at about 2550 F. at one atmosphere, and at lower temperatures at reduced pressure. The silicone resins or other polymers which may be used as separators vaporize at considerably lower temperatures than the other materials.

What is claimed is:

1. A pressure vessel formed of a composite laminar structural material, said structural material comprising a plurality of thin films of metal, the metal of each of said thin films being selected from the group consisting of chromium, iron, aluminum, nickel, and titanium and being approximately 400 to 1000 angstroms thick; said plurality of thin films alternating with layers of silicon monoxide approximately 50 to 400 angstroms thick; said layers of silicon monoxide spacing apart said thin films of metal to preclude metallurgical contact therebetween.

metal of said thin films is aluminum.

5 6 2. A pressure vessel as recited in claim 1, wherein the 3,271,561 9/1966 Fiedler 117106 X 3,310,424 3/1967 Wehner et a1 117-106 References Cited FOREIGN PATENTS 952,043 3/ 1964 Great Britain.

UNITED STATES PATENTS Petriello 117- 7.1 X ALFRED L. LEAVITT, Primary Examiner 9 X A. GOLIAN, Assistant Examiner 183531361 et a1. 117--107.2 X 10 Us. CL X R Bud, et a1 117.406 117-71, 106, 107.1; 161207, 213; 1164-46; 220-3 Cummin et a1 117-217 

